When purchasing a boat propeller there are a lot of options and parameters to choose from for optimal performance. One of these options is having the propeller blade trailing edges formed with a tight bend or “cup”. There is often a lot of uncertainty regarding both when a cupped propeller should be used and what cupping actually does. Hopefully this article can help shed some light on the topic and provide everyone with a better understanding on how Michigan Wheel uses this technique to provide its customers with propellers that will perform optimally.
Cupping or adding a cup is a pretty simple geometric change of the trailing edge of the propeller blades. This is done by bending the propellers blades’ trailing edge towards the pressure face (blade surface facing aft) thus resulting in an edge with a noticeable curve. This curved area is called “cup”. In appearance it looks as if the trailing edge is bent into a small radius. This can be felt by running your finger from the leading edge toward the trailing edge. It will be felt at the very end of the blade chord length. This amount of cupping can vary both in how large the bend is and where on the blade trailing edge it is located. For outboard propellers, our engineers develop a particular cup for each type of propeller and it is then accurately manufactured into all of the same parts. For inboard propellers, there may be several different degrees of cup available for a particular propeller style.
Cupping the propeller provides a lot of benefits for relatively small changes to the blade form. Adding a cup changes the effective or nominal pitch by essentially adding camber to the blade shape. This camber helps reduce cavitation and increase performance. Being that it is such a small change, with little impact on the rest of the blade design, we can apply this change to existing standard propeller designs without significant changes to the patterns or tooling used to make these propellers.
One of the common rules of thumb regarding correct propeller cup is:
When using a cupped propeller, the design pitch value should be reduced by 1-2 inches compared to the non- cupped propeller.
This can be a little confusing to most people who are not entirely familiar with propellers. When applying a cup a reduction in propeller pitch is recommended to keep the effective pitch the same. The small detailed change in the blade trailing edge shape is what helps the propeller perform better.
Below are a few common questions that we get regarding propeller cup. Hopefully this information will help you to better understand the use of cupped propellers. If there are any more questions, your dealer or the customer service representatives at Michigan Wheel would be happy to assist you!.
Which Michigan Wheel propellers have cup?
The Michigan Wheel advanced propeller designs have trailing edge cup designed into most of our Outboard and SternDrive propellers including the stainless steel prop models. Almost all of our standard series inboard propeller designs may be specified with or without cup, depending upon the particular boat application. Many of our custom inboard propellers are not designed with cup, we utilize a more advanced blade geometry that provides the same benefits as cup. If a customer decides for their particular application that cup would be desired, it can easily be added later.
Why don’t I just add a cup to the already correctly pitched prop?
This would not necessarily always work. This would result in a propeller with effective Higher Pitch. This would in turn reduce the maximum engine RPM obtained at wide open throttle. If your engine is already just reaching the minimum recommended maximum engine RPM then adding cup would result in an overloaded engine and could potentially result in poor boat performance.
Can Cup be added or removed to my propeller instead of buying a new prop?
Yes, propeller repair shops are very familiar with adding, removing, or changing the amount of cup in a propeller.
Is a propeller cup recommended for all applications?
No but is always something that can be added at any prop repair facility later if necessary. We recommend propellers with cup for high cavitation applications and or applications where the vessels speed it above 30 knots or 34 mph. Using propellers with trailing edge cup are most beneficial for planning hull boats running at higher speeds.
Does cupping the propeller help with fuel efficiency?
Maybe. Propeller Cup helps with propeller efficiency. If the boat is operated at certain speeds where propeller efficiency is improved due to cup, then there should be an increase in fuel efficiency. So, for example, if a fishing boat spends the majority of it’s time trolling, at low speeds and engine power, then the benefits of propeller cup will not likely be noticed. However, if this boat spends a significant amount of time running at higher speeds, say to get to the fishing grounds, then cupped propellers would likely help improve the fuel efficiency over a non-cupped propeller. Having the right propeller for your vessel is the best way to ensure you are getting the most out of your engine and the best fuel efficiency.
Which is better Cupped Prop vs Custom?
There are a lot of great hydrodynamic tools out there to evaluate propeller performance. When doing custom propeller design work we run multiple simulations iterating over and over again trying to achieve optimal performance. We do this by using propeller blade designs with variable pitch or progressive pitch characteristics and adding specific camber (curvature) into our blade section designs. This is a very similar concept to the propeller cup. Only we are performing this over the entire blade surface out to the propeller blade tip.
Can Cup be applied to a Stainless steel propeller?
Yes it can be applied but it is a lot more difficult than applying it to a bronze or NiBrAl propeller. Michigan Wheel casts cup into our outboard Stainless steel propellers during the manufacturing process. Inboard propellers made in Stainless steel may be cupped but the actual process of forming the trailing edge cup is more difficult due to the material properties of Stainless steel. Therefore, the cost of cupped Stainless steel propellers may be higher. You may need to speak to a propeller shop about the specifics.
Can too much cup be applied?
Yes. There are multiple factors to consider including, engine loads, propeller material properties and the blade design trailing edge thickness. The cup application includes bending the trailing edge to the pressure face. If this bend is too excessive or the blade edge is too thin this could lead to cracking on the trailing edge. This is obviously less desired and would require additional repair.